He added that the attack was possibly a "politically motivated crime".

The "red-shirt" government supporters - who shut down Bangkok in 2010 - have for the most part stayed away from these protests. But observers fear that violence could erupt if a trigger brought them out onto the streets.

In Bangkok, meanwhile, there was little change seen on the streets in the first few hours of the state of emergency, with anti-government protesters continuing their blockades in the city centre.

Image copyrightAFPImage caption
This state of emergency will be run by police, not the army, the government says

In the past, emergency decrees in Thailand have usually meant deploying the army to impose order on the streets, says the BBC's Jonathan Head in Bangkok.

This time, the government says emergency rule will be run by the police, who until now have been told to avoid confrontations with the protesters and have performed poorly when attempting crowd control, our correspondent adds.

Meanwhile, the US State Department issued a statement urging "all sides to refrain from violence, exercise restraint, and respect the rule of law".

"We encourage all involved to commit to sincere dialogue to resolve political differences peacefully and democratically," the statement from spokeswoman Marie Harf added.